Ion channel activity at the nerve terminal determines presynaptic action potential shape and Ca2+ entry and thus plays a pivotal role in the regulation of synaptic transmission. This is particularly important in the presynaptic terminals of the neocortex, due to this brain region's role in mediating higher neurological function under normal conditions and during disease states. We have recently developed a technique that permits electrophysiological recording from single, acutely isolated rat neocortical nerve terminals. The long-term objective of the laboratory is to answer questions about the physiological and pathophysiological regulation of synaptic transmission by directly studying neocortical, presynaptic ion channels with this technique. An increase in intracellular [Ca2+] ([Ca2+]i) is a critical signal at the synapse where it triggers exocytosis, plasticity and gene expression. Much more is known about signaling downstream of changes in intracellular Ca2+ than about the impact of changes in extracellular [Ca2+] ([Ca2+]o). Yet [Ca2+]o is likely to undergo significant changes as a result of electrical activity. The driving hypothesis for this proposal is that a decrease in synaptic cleft [Ca2+] is an important signal which regulates synaptic efficacy. We have recently discovered a novel, Ca2+-based signaling pathway in neocortical nerve terminals, comprised of a voltage sensitive non-specific cation (NSC) channel activated by decreases in [Ca2+]o. This interesting finding poses a number of questions: what is the mechanism by which changes in [Ca2+]o are detected and transduced to alterations in membrane conductance? Is the Ca2+ sensor-NSC channel signaling pathway modulated by other agents at the nerve terminal? What is the physiological impact of Ca2+ sensor-NSC channel signaling pathway on synaptic transmission? To answer these questions we plan to use a combination of electrophysiological, pharmacological and immunochemical techniques to: 1. Identify the constituents of the Ca2+ sensor-NSC channel signaling pathway. 2. Determine physiological modulators of Ca2+ sensor-NSC channel signaling pathway. 3. Determine the role of the Ca2+ sensor-NSC channel signaling pathway in synaptic transmission. The goals of this proposal are to understand the mechanism by which [Ca2+]o modulates ion channel activity in the synapses of the cortical nerve terminals and to determine how this Ca2+ sensor-NSC channel signaling pathway impacts synaptic transmission in the neocortex.